It has been known for over a century that small electrical currents applied to the brain can activate neurons, trigger movements, and change behaviors. Recent years have seen an explosion of applications of brain stimulation to the treatment of neurological disorders. Worldwide, more than 60,000 deaf patients have recovered hearing through cochlear implants. Similarly, deep brain stimulation has proven highly valuable in the treatment of intractable movement disorders including Parkinson's. Today, its use is being expanded to a variety of severe, otherwise untreatable disorders, including refractory depression, obsessive-compulsive disorders, and even to promote recovery of function in stroke patients. Although electrical stimulation is routinely used today in human brain implants, we do not know how electrical stimulation acts to change neuronal activity in the brain. There is considerable debate, for example, as to whether deep brain stimulation activates activity in the targeted area or whether it suppresses it. The fundamental issue to be addressed, therefore, is how populations of cells respond to electrical stimulation. Are all the cells in the targeted area activated? Are cells recruited one by one as current is increased, or is there a threshold at which a set of cells fire in response to stimulation? The new technique of in vivo two-photon imaging of calcium signals provides a unique opportunity to answer these questions. With this technology, we can image the activity of thousands of neurons simultaneously as electrical stimulation is applied. We can thus measure how virtually every neuron in a local volume is affected by stimulation, and provide basic answers to the questions posed above. We have two specific aims. First, we will use single electrodes to stimulate in the visual cortex and assess the spatial distribution of cells that are activated (or suppressed) by microstimulation with various currents and pulse trains. Second, we will stimulate through a multi-site electrode with closely spaced contacts (50 5m separation), and ask how stimulation effects vary as the location of the current injection is varied. Does stimulation at different sites activate different cell populations or do these populations overlap? Do responses from different sites sum linearly or are there nonlinear interactions between these responses? These simple protocols will help ground a 50- year literature on cortical stimulation. More generally, this work will establish protocols for the use of calcium imaging to calibrate the effects of stimulation in neural circuits. PUBLIC HEALTH RELEVANCE Electrical stimulation of the brain is used in cochlear implants to provide auditory sensation to the deaf, in experimental visual prostheses to provide visual sensation to the blind, and in treatments to relieve patients from the symptoms of Parkinson's disease. Our proposed studies will use a new imaging technique to visualize at high-resolution brain activity during stimulation, which has previously been impossible. Measuring how neurons respond to electrical stimulation will help devise better treatments for diseases of the brain.